1. Field of the Invention
The present invention generally relates to a method and apparatus of a forward traffic channel power control for a code division multiple access (hereinafter referred to as a CDMA) wireless local loop system. More particularly, it relates to a method and apparatus of a forward link power control in a CDMA wireless local loop system.
2. Description of the Conventional Art
Generally, a forward link (i.e., a wireless link from a base station to a mobile station) power control in a mobile communication system using CDMA is performed at every traffic channel which transmits traffic information to each mobile station.
FIG. 4 is a conceptual diagram illustrating a general CDMA wireless local loop system.
As shown in FIG. 4, a mobile station 411 includes at least one terminal (e.g., a telephone set and/or a terminal for data service) in a cell 410, and has all functions which are needed to achieve communication between a base station 412 and the mobile station 411.
The base station 412 receives all kinds of control information necessary for the mobile station 411's communication, and traffic information after setting a call. In addition, the base station 412 transmits various information to the mobile station 411.
Likewise, a base station 422 and a mobile station 421 perform the same function as the above stations 412 and 411, respectively.
To compensate a propagation environment and a path loss caused by a distance from the mobile station to the base station, the base stations 412 and 422 control a forward link (i.e., link from the base station to the mobile station) traffic channel power when operating in a cellular communication.
In order to maintain a communication quality of the mobile stations being used in the base stations 412 and 422, the base stations 412 and 422 respectively control a transmission power of the forward traffic channel in consideration of the propagation environment and the path loss. Accordingly, a large quantity of signal power are transmitted to the mobile stations 411 and 421 respectively located in the vicinity of the edges of the cells 410 and 420.
However, if the traffic loads increase in the cell 420 served by the base station 422, the mobile station 411 being communicated with the base station 412 becomes receive more strong interference signal from the base station 422, thereby degrading the communication quality.
When the mobile station 411 being communicated with the home base station 412 receives more strong signal from the adjacent cell 420, the mobile station 411 performs a hand-off function toward the adjacent cell 420, thereby continuously preserving the cellular communication. As a result, it can reduce the influence of interference signal applied from the adjacent cell in a boundary portion between the cells.
However, the wireless local loop system not having the hand-off function can not switch its communication channel to the adjacent cell, in case the interference is increased in the forward link, thereby exacerbating the communication quality.
Accordingly, if the traffic loads (i.e., the number of busy mobile stations) of the adjacent cell 420 increase, the interference power of the forward link of the mobile station 411 being busy in the predetermined cell 410 becomes increased. As a result, the communication quality of the mobile station 411 positioned in the vicinity of the edge of the adjacent cell 420 becomes lowered, thereby causing a drop of the call.
In particular, the interference signal's variation in a wireless local loop system providing data traffic as well as voice traffic simultaneously is larger than that in a single traffic environment (i.e., wireless network which provides only voice traffic or data traffic), thus the aforementioned problem becomes more serious.
A conventional power control method about the forward traffic channel controls the transmission power of the forward traffic channel, not in view of the interference quality which comes to the adjacent cell, but in view of the traffic quality of the mobile station which is busy in the home cell.
The reason why the conventional power control method controls the transmission power is that the interference influence varied in response to the traffic variation can be compensated by the hand-off function if a proper communication quality is ensured.
That is, in order to enhance the communication quality of the mobile station being busy at the edge of the home cell in the digital mobile communication system, the base station should transmit more power toward the forward traffic channel. In this case, a corresponding mobile station maintains a proper traffic quality, and causes more strong interference signal on the forward link.
However, if there is a cell having a small quantity of traffics in the adjacent cells, a channel propagation environment toward the adjacent cell is better than a channel propagation environment of the home cell, so that the mobile station can perform the hand-off function.
When the interference signal level becomes higher due to the increased traffics within the cell, the conventional digital mobile communication system lowers a power control threshold value, or uniformly reduces the traffic channel transmission power of all mobile stations being busy in the home cell as much as a predetermined value, thereby lowering a call drop rate. However, this method has a disadvantage which lowers the communication quality of the mobile station.